1. Field
Embodiments herein generally relate to liquefied gases. More particularly, the embodiments relate to methods and apparatus for pumping liquefied gases from storage containers.
2. Description of the Related Art
Liquefied gas is stored in cryogenic storage containers prior to use or during transportation. The more common liquefied gases, include liquefied natural gas (LNG), liquefied petroleum gas (LPG), liquefied energy gas (LEG), liquefied ethylene, natural gas liquid, liquefied methane, liquefied propane, liquefied butane, and liquefied ammonia. Such liquefied gases are extremely volatile and flammable and therefore require special care. Systems for pumping liquefied gases from the storage container have utilized a two pump system, a smaller pump submerged in the liquid within the container and a larger, high head pump located outside the container.
FIG. 1 depicts an illustrative prior art liquefied gas container system. An in-tank pump 30 is enclosed within a column or casing 20 located within the tank or container 10. A booster pump or second pump 80 is located outside the container 10 and is used to boost the fluid pressure to end-use requirements. A sealing gland 150 is placed on top of the container 10 to prevent gas and fluid leakage during pumping.
A foot valve 40 is typically located at the bottom of the column 20 beneath the in-tank pump 30 to regulate suction flow to the in-tank pump 30. The foot valve 40 is normally closed and is actuated to an open position when the in-tank pump 30 rests thereon. Routine maintenance and service of the foot valve 40 is an inherent problem and challenge, especially if the foot valve 40 were to fail. Most foot valves fail open and the fluid is allowed to flow into the casing 20. If a foot valve were to fail closed, which is very rare, large volumes of product might be sealed into the container 10, making recovery difficult. In the field, when foot valves like valve 40 fail to open, typically no attempt is made to repair the valve since the fluid can still enter the pump and removal operations can continue.
The in-tank pump 30 is typically a one or two stage impeller pump that produces relatively low discharge pressures such as below about 15 bar. The first pump 30 transfers the liquid from the inside of the container 10 through the column 20 to a send out pipe 70 in fluid communication with the second pump 80.
The second pump 80 is known as a booster pump. The second pump 80 is located outside the container 10, as shown in FIG. 1. The second pump 80 is typically a multi-stage, high pressure pump. The second pump 80 may have discharge pressures of about 85 bar or more. From the second pump 80, the liquefied gas enters a distribution system (not shown) for further processing, vaporization, and/or use.